Spark plug and method of making same



July 14, 1959 A. 'CANDELISE SPARK PLUG AND METHOD OF MAKING SAME Filed July 8, 1955 ATTORNEY fig/ gp 2 g rey 232215552 United States Patent O l 2,894,315 SPARK PLUG AND METHOD OF MAKING SAME Alfred Candelise, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application July 8, 1955, Serial No. 520,734 2 Claims. (Cl. 29-2512) time be both reliable and have a relatively long life. One

of the problems encountered is that of plug fouling as a result of. the combustion process occurring within the cylinder. Conventional type plugs utilize a relatively large air gap in combination with a high voltage ignition system. Such systems are susceptible to failure as.a result of the plug fouling.

It has been proposed to use a spark plug having a relatively small discharge gap and being so constructed as to provide a low tension discharge path for use in a low voltage ignition system. By using high energies, a hot spark may be obtained such as will inhibit the tendency towards plug fouling normally encountered in conventional plugs. Likewise, by reason of the use of a low voltage system, any fouling which does occur does not have as harmful an effect as that occurring in high voltage systems. These low tension or creep gap plugs comprise a pair of electrodes separated by an insulator body having the surface abutting the spark gap of such a Patented July 14, 1959 electrode 11 secured to the end surface of the shell 3 and with a semi-conductive material 13 provided on the end face 15 of the insulator 5. The semi-conductive material 13 is positioned in abutmentwith the ground electrode 11 and the firing end of the center electrode 7 and adjacent the spark gap thus distinguishing the plug 1 as a low voltage type spark plug.

As can be seen from the drawing, the shell 3, having the ground electrode 11 welded or otherwise securely attached thereto, is threaded on its lower end for ready assembly in the engine. The shell 3 is provided with an internal ledge 17 formed between the upper large diam eter portion 19 and the lower small diameter portion 21. The insulator 5 is formed with an enlarged center portion 23 having an upper shoulder 25 and a lower shoulder 27 which shoulders cooperate with the shell 3 in securing the insulator therein. The lower shoulder 27 is positioned on the ledge 17, a sealing gasket 29 being positioned therebetween. The gasket 29 may be formed of any suitable heat resistant, relatively soft metal, such as I nickel or nickel-steel alloy and serves to provide a seal spectively positioned a terminal screw 37 and the small nature as to form a path between the electrodes for permitting a low voltage discharge. Such insulators have been disclosed as having a coating or layer of semiconductive material which is sintered or otherwise applied to theinsulator surface and as being formed of such materials as metals, metal oxides, metal carbides, carbon and various mixtures of one or more of these compounds with ceramic materials. However, it has been found in practice that such low tension spark plugs and the methods for making the plugs were both complex and expensive.

One of the objects of this invention is to provide a long life and eflicient spark plug adapted for use at relatively low voltage to produce a hot spark. Another object of this invention is to provide a low tension spark plug wherein the conditioned surface of the insulator adjacent the spark gap is substantially non-porous and is tightly adherent to adjacent plug portions to form a gas-tight seal. Another object of this invention is to provide a novel method for making spark plugs. A further object of this invention is to provide a simple and inexpensive method for makinglow tension spark plugs.

These objects are attained in accordance with my invention by providing a spark plug wherein the center electrode is used as the hot-pressing ram for both forming the seal within the insulator centerbore and for forming the creep gap insulator surface.-

Other objects and advantages of my invention will be apparent from the following description and from the drawing in which Figure 1 is a vertical cross-sectional view of a low voltage spark plug formed in accordance with my invention; and Figure 2 is a vertical cross-sectional view of another embodiment of a spark plug having a firing end construction different from that of Figure 1.

Referring now to Figure 1 there is shown a spark plug 1 having a shell 3 in which there is positioned in sealing relationship therewith an insulator 5 having a center electrode 7 sealed within the centerbore 9. The lower diameter portion 39 of the center electrode 7. The lower end of the terminal screw 37 is formed in an enlarged positioning flange 41 which is electrically and physically interconnected with the threaded or knurled or otherwise roughened end portion 39 of the electrode 7. The interconnection is achieved by means of a glass-like conductive seal 43 comprising a mixture of glass and conducting metal powder such as copper orjnickel, such conducting seals being well known in the art.

The semi-conducting material 13 is positioned on the lower end 15 of the insulator 5 and in the annular space formed between the insulator and the center electrode 7 and the shell 3 and extends flush with the lower surface of the ground electrode 11 and the center electrode 7. The semi-conductive material 13 serves not only as the determining factor for the electrical characteristics of the plug but also serves as a sealing material for establishing a gas-tight seal between the center electrode 7, the insulator 5 and the shell 3. The semi-conductor materials found to be most suitable in the relationship described are the stanno-titanate compositions described in copending applications Serial No. 357,907 filed May 27, 1953 and Serial No. 406,338 filed January 26, 1954, through various other materials, i.e., carbides such as silicon carbide, germanium, various sulfides and such conductor materials as copper, nickel, etc. in admixture with glass, may also be used.

In accordance with my invention, the plug shown in Figure 1 may be formed simply and inexpensively in the following manner. The ground electrode 11 is first welded or otherwise secured to the lower end of the shell 3. A center wire sub-assembly is then formed by positioning the terminal screw 37 in the small diameter portion 33 of the insulator centerbore 9 and threading or otherwise seouring the terminal nut 45 on the upper end thereof. A preform of the semi-conductive material 13 is then positioned within the shell 3 on the inner surface of the ground electrode 11. The center wire sub-assem bly is then positioned within the shell 3 with the lower sealing gasket 29 positioned between the .sealledge 17 and the insulator shoulder 27. The thus formed assembly is positioned within a jig or die or other supporting fixture and the conductor seal material 43 is loadedinto the large diameter portion 35 of the centerbore. Upon positioning the center electrode 7 on the seal material and within the centerbore, the assembly is heated to a temperature suflicient to soften the glass seal and semiconductor materials, a temperature of from 1200 to 2000 F. being found suflicient for most materials. A generally axial pressure of 300 pounds per square inch or higher is applied to the electrodes 7 and 11 to hot press the glass seal'and semi-conductor materials into position, the assembly being permitted to cool while under pressure in order to permit the materials to solidify. The excess creepage material 13 is then cleaned off the face of the electrodes in any suitable manner such as by grinding. The top gasket 31 is then positioned on shoulder 25 and the relatively thin upper portion of the shell 3 is rolled over onto the gasket to complete the assembly of the insulator within the shell.

Though further precautions may be taken to insure the seal of the insulator within the shell, i.e., as by Cico Welding or collapsing the shell in an annular portion between the top thereof and the sealing ledge 17, this is not preferred since such an operation may shatter the glassy semi-conductor material 13 unless rigid and expensive controls are utilized. In addition, it is considered that no further sealing precautions need be taken since in addition to the presence of sealing gasket 29, the semi conductor material 13 adheres closely to the metal electrodes and shell and to the ceramic insulator and thus forms a gas-tight seal at the firing end of the plug.

In the embodiment shown in Figure 2 the semi-conductor material 13, similar to that described with reference to Figure 1, is already an integral part of the firing end of the insulator. The semi-conductor material 13' may be applied to the end of the insulator during the insulator molding operation thus forming a monolithic structure or the material may be applied as a paint or spray which diffuses into the insulator. In any case, the structure and method of assembling a creep gap plug utilizing such an insulator are greatly simplified. Likewise, in those instances where the semi-conductor material is not used in any form, the resulting plug is a structurally simple and inexpensively formed device which is readily adaptable to mass production fabrication and use in the automotive applications.

In accordance with my invention, an insulator subassembly is first formed by positioning the terminal screw 37 in the small diameter portion 33 of the centerbore 9 and threading the terminal nut 45 on the upper end thereof. The sub-assembly is then completed by loading the conducting seal material d3 into the large diameter portion 35 of the centerbore 9, positioning the center electrode on the seal material and within the centerbore, and by heating the components thus assembled to a temperature sufficient to soften the glass seal material. Upon softening the seal material, generally axial pressure is applied to the center electrode 7 to hot press the material within the centerbore-and thus form an electrical and physical interconnection between the electrode and the terminal screw 37. Pressure is maintained on the electrode until the seal material has cooled sufficiently to solidify. The thus formed sub-assembly is then positioned within the shell 3 with the sealing'gasket 29 positioned between the ledge 17 and the shoulder;27. The upper gasket 31 is then positioned on the upper shoulder 25 of the insulator and the top of the shell 3 is rolled over onto the gasket, the seal of the insulatorS within the shell 3 being assured by Cico welding to collapse the thin wall annular portion 47 of the shell. The ground electrode 11 is then properly located and brazed or welded within the end of the shell andthe shell lip 49 is crimped thereover. As can be seen from the drawing,., the firing. end. of the center electrode andground electrode 11' are adjacent the end face of the insulator 5 and are spaced from each other to form the spark gap. Where desired, a heat resistant metal gasket, i.e., nickel, nickel alloy, silver, etc., may be positioned between the insulatorface and the firing end of the center electrode and between the insulator face and the ground electrode. 'Such gaskets .serve to compensate formuevenness in the surface of the-insulator and-the electrodes and thus assure a gas-tight seal between the. parts.

It can thus be seen that I have provided a simple and inexpensive method for forming spark plugs of either the conventional type or the low voltage creep gap type wherein the electrodes themselves serve as hot-press rams thus forming a more positive seal between the electrodes and the elements. While I have described my invention in terms of two modifications, various other embodiments will be apparent from the above description and it is intended that. such variations be within the scope of my invention as defined by the claims which follow.

I claim:

1. A method for forming a low voltage spark plug of the type comprising an elongated insulator within a metal shell having an inwardly directed flange the lower end thereof constituting an outer electrode, said insulator having a center electrode positioned therein and providedwith an outwardly directed circular firing head to define an annular spark gap with the outer electrode, said method comprising the steps of positioning a preformof semi-conductive material on the inner surface of said outer electrode to extend beyond the annular surface thereof defining the gap, positioning a terminal screw within the centerbore of the insulator, locating saidinsulator within said shell, loading conducting seal material into saidcenterbore, positioning the center electrode within said centerbore with the inner surface of the firing head extending over said preform, applying heat to softensaid seal material and said semi-conductive material, applying substantially axial pressure to said-center electrode to hot press said materials, cooling the insulator and shell assembly, to solidify said materialswhile maintaining the application of pressure, and completing the assemblyof said insulator Within. said shell, saidsemiconductive material being forced by said application of pressure into the annular space of the spark gap and into direct contact with the electrodes and insulator.

2. In a method for forming a low voltage spark. plug of the type comprising an electrical insulator Within a metal shellthe lower end of which forms an annular outer electrode, said insulator having a center electrode positioned therein and provided with a firing head extendingoutwardly and spaced from the outer electrode to define a spark gap with the outer electrode, the steps of positioning semi-conductive material Within and .at the lower end of said shell, positioning said insulator Within said shell, positioning the centerelectrode within the centerbore with the firing head overlapping the semiconductive material, applying heat to soften said isemiconductive material, applying substantially axial pressure to said center electrode to hot press said semi-conductive material into said spark gap, cooling the insulator. and

shell assembly to solidify said semi-conductive material,

and completing the assembly of said insulator within said shell, said semi-conductive material being in direct contact with the. electrodes and insulator.

References Cited in theiile of this patent UNITED STATES PATENTS 1,913g575 Vollmar June 13, 1 933 2,267,571 McDougal Dec. 23, 1941 2,657,248 Smits Oct. 27, 1953 2,684,665 Tognola July 27, 1954 2,699,158 Purdy Jan. 11, 1955 FOREIGN PATENTS 724,211 Great Britain Feb. 16, 1955 

